Hi, I had the same problem, but then I realized that I didn't remember to copy the machinekey from the old web.config file to the new one before starting the upgrade. After copying the machinekey to the new file, everyting was back to normal.

If you didn't remember to make a backup copy of your web.config file before starting the upgrade, and therefore doesn't have access to the old machinekey, you could try the following. Start by registering a new user, and make sure to remeber the password. Then open your database and open the table "aspnet_Membership". Copy the values in the fields "Password" and "PasswordSalt" from the newly created user to your "Host" and "Admin" accounts. After this, your "host" and "admin" accounts should have the same password as the newly created user, and you should be able to login to the portal again.

Hopes this helps you!

Thank you very much!
The Wokaround you provided is a good idea. Maybe someone could put this into a FAQ?

In my case it was the encryption Keys!
I'm so sorry for wasting your time.... i kept them in the previous upgrades but this time i forgot.